The invention relates to a method of producing cement clinker and electricity, comprising feeding cement raw mix and hydrocarbon in a circulating fluidized bed, calcining the cement raw mix in the circulating fluidized bed and producing steam therein, discharging hot bed material in a rotary kiln, clinkering the calcined material in the rotary kiln and subsequently cooling the clinker, further comprising the gas and the solids out of the fluidized bed entering a cyclone, the solids being separated therein being returned to the bed, whereby part of those solids being first cooled down in a solids heat exchanger producing steam, whereas the gas and the fly ash escaping the cyclone being passed through at least one heat exchanger and through a filter, further comprising the produced steam being fed to a steam turbine island comprising a steam turbine being drivingly connected to a generator.
U.S. Pat. No. 4,425,163 describes a method for producing cement clinker, in which raw cement powder is calcined to a level of at least 95% in a circulating fluidized bed and then clinkered in a rotary kiln. The circulation system comprises a fluidized bed reactor, a cyclone separator and a recycling duct. The carbonaceous fuel supplied to the calcining fluidized bed reactor is burnt near stoichiometrically in two stages with fluidizing gas and secondary gas. This method requires a preheating of the raw material in a suspension type heat exchanger.
A method of producing cement clinker and electricity as described above in the xe2x80x9cfield of the inventionxe2x80x9d is known from the article xe2x80x9cChallenges of circulating fluid bed reactors in energy and raw material industriesxe2x80x9d by Lothar Reh, published in DECHEMA Deutsche Gesellschaft fxc3xcr chemisches Apparatewesen, Chemische Technik und Biotechnologie e.V., Frankfurt am Main. This article is based on a lecture held on the 6th International Conference on Circulating Fluidized Beds, in Wxc3xcrzburg, Germany on Aug. 22-27, 1999. This article describes trigeneration of cement, electricity and heat from high-ash coal using Circulating Fluidized Bed-Technology. Limestone and high-ash coal are separately prepared and ground before feeding them into the boiler. These two basic raw materials are proportioned online. Sulfur is completely bound into the bed material, which has the chemical composition of clinker. The hot bed material is discharged directly into a small rotary kiln. This direct use of ash substituting the clay component in cement avoids disposal of ash completely. This known method requires high performance cyclones in order to catch and recycle the pulverized limestone and/or lime and the grinding of the high-ash coal prior to its entry in the boiler.
Accordingly, the object of the invention is to provide a novel method and plant for integrating a cement clinker-buming unit into a hydrocarbon, especially coal-fired power plant, using circulating fluidized bed technology.
This is achieved, according to the invention, by following means
feeding part of the cement raw mix in pulverized form,
designing the cyclone system so as the lime-rich fly ash obtained from calcination of limestone from the pulverized part of the cement raw mix escapes the cyclone system,
feeding the predominant part of the remaining part of the cement raw mix in crushed form for forming bed and circulating material,
and grinding the hot bed material as well as additives before blending them with lime being separated in the filter, obtained from calcination of limestone in the cement raw mix.
The advantages of the invention are to be seen in a significantly smaller number of cyclones leading to lower capital cost, because the cyclone system may have a larger cut size than the above mentioned prior art devices. It has a lower pressure drop, which leads to lower power consumption and lower operation cost. It also has a higher reliability due to the absence of fines in the material collected in the cyclone, which becomes easier to discharge. Another advantage is seen in the fact that there is no need to grind the fuel feed. Lastly the nitric oxide emissions are lower.
The discharged hot bed material may be cooled before grinding. This allows reliable operation of the downstream grinding equipment and gives an opportunity for further steam production.
The heat exchanger cooling the flue gas and the fly ash may be completely utilized for preheating air that is fed to the bed as fluidizing and overbed air. This requires only one piece of equipment, resulting in a high temperature driving force for steam generation in the remainder of the system. Another advantage is the short residence time of the gases and the fly ash in the heat exchanging apparatus, thus avoiding recarbonation of the lime to CaCO3.
The at least one heat exchanger may also be a steam producer followed by the air heater. In this case, it is preferable to cool the gas and the fly ash escaping the cyclone and entering the steam producer at a fast rate in order to avoid recarbonation.
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing, which illustrates diagrammatically an exemplary embodiment of the invention with coal as hydrocarbon. Only the elements essential for understanding the invention are shown. Arrows illustrate the flow direction of the working media.